Is a render farm suitable for using high-res stills?
That will depend on the render farm you plan to use. Normally a render will generate a complete image which means animations can easily take advantage of multiple computers as each computer can render a single frame from the animation.

It is possible to cut up a single render into multiple pieces that can later be stitched together into a larger image, this will allow multiple computers to work on the same larger final image. Blender's render border option can be used with a python script like in this answer, it is possible to use imagemagick to automatically stitch the pieces into one image.

Some commercial render farms include an option to do this, Render.st is one example. If you are creating a personal render farm then chances are you will need to add this ability yourself.